1. Field of the Invention
The invention relates generally to devices for the prevention of pulmonary embolism and, more particularly, the invention relates to a filter device for placement within the inferior vena cava that is operable to prevent embolus migration.
2. Description of the Related Technology
Blood coagulation, or clotting, is the transformation of blood from a fluid initially into a semi-solid gel and ultimately into an organized, fibrinous solid. Coagulation is the body's mechanism to control and stop unwanted bleeding. Despite the desirable and necessary effects of blood clotting, several undesirable effects may result from clotting which can lead to serious medical disorders and, in some instances, death. One example of such an undesirable effect of clotting is deep vein thrombosis (DVT), the formation of blood clots within the veins of the legs. DVT which is not diagnosed and successfully treated can result in the migration of blood clots to the pulmonary arteries in the lungs, arresting blood flow through the lungs (pulmonary embolism). Pulmonary embolism is one of the most common causes of death for hospitalized patients, and results in more than 200,000 deaths annually in the United States.
The disease process in which blood clots form within a vein and subsequently migrate through the venous system to the pulmonary circulation is called venous thromboembolic disease. Numerous factors, acting independently or simultaneously, can cause clot formation within a vein and, potentially, thromboembolism: (1) damaged or irregular surfaces on the inside of the vein can lead to thrombus formation, (2) imbalances in the body's natural clotting-lytic (clot-dissolving) systems can trigger clot formation, (3) slowly-moving blood or blood passing through an area of turbulence within the vein, such as that in blood pooled in damaged leg veins, is likely to clot, and (4) the release of clotting factors into the blood from traumatized tissue is likely to trigger widespread clotting.
Various methods have been developed to treat venous thromboembolic disease. Anticoagulant drugs ("blood thinners") slow the process of blood coagulation. Anticoagulants such as heparin and its low-molecular weight variants as well as warfarin, are the front-line treatment for deep vein thrombosis and pulmonary embolism, and are largely successful. Some patients, in particular those with extensive clotting which threatens to damage the deep veins, are treated with enzymes known collectively as thrombolytics which actively break down existing clots. However, some patients cannot be successfully treated with anticoagulants or thrombolytics because clots continue to form in spite of adequate dose levels of these drugs. Other patients cannot be given anticoagulants or thrombolytics due to other medical conditions.
For patients in whom anticoagulation has failed or to whom it cannot be administered, it was necessary to develop alternative treatments to prevent pulmonary embolism. One of the earliest surgical solutions was ligation of the common femoral vein to prevent the migration of clots from the large veins in the upper leg to the lungs. Then, in the mid 1940s, Oschner and DeBakey and O'Neil proposed ligating the inferior vena cava (the large central vein in the abdomen) in order to prevent emboli from migrating from the legs or pelvic area to the pulmonary circulation. While effective in preventing pulmonary embolism, this technique resulted in complications due to diminished blood return to the right heart.
To avoid employing ligation techniques, vein clips and, subsequently, blood filters were developed to prevent pulmonary embolism. Vein clips are intended to be placed around the vein during an open surgical procedure, and such clips reduce the area within the vein available for blood flow to one or several small channels to prevent clots from passing through to the pulmonary circulation. Blood filters are implanted within the vein, typically the inferior vena cava, and are intended to trap large blood clots while allowing blood to pass freely through the filter around the clot. In most cases trapped blood clots will normally dissolve over time.
Blood filters (vena cava filters) are placed within the inferior vena cava lumen from a variety of peripheral vein access sites, for example, the jugular or femoral veins. An early example of such a filter was the Mobin-Uddin (MU) umbrella filter, which was developed and made available by American Edwards Laboratories in Santa Monica, Calif. in the 1970s. The Mobin-Uddin umbrella was composed of six flat ELGILOY spokes radiating from a hub and partially covered by a web designed to capture blood clots. MU filters were introduced into the body via a cutdown of the jugular or femoral vein and subsequent passing of a catheter through the access site to the filter implant site in the infrarenal inferior vena cava. While this method was an improvement over previous methods, the MU filter was associated with a high incidence of occlusion of the inferior vena cava, in which blood flow through the vena cava was completely obstructed.
In the mid 1970's, the Kimray-Greenfield (KG) vena cava filter was introduced. The original KG filter is conical in shape and is composed of six stainless steel wires equally spaced with its apex cephalad. Although the filter was originally placed using a local cutdown of the jugular or femoral vein, it was later adapted to be inserted percutaneously. The KG filter is designed to capture clots 7 mm or greater in diameter, holding the clots in the infrarenal vena cava until the body's own lytic system dissolves the clot. The principal drawbacks of the KG filter are the possibility of tilting and filter migration, often related to a failure to open, or untimely ejection of the filter from the introducer.
Subsequent versions of the so-called Greenfield filter were developed to reduce the size of the introducer catheter to facilitate percutaneous introduction. Other vena cava filters were introduced in the United States in the late 1980s, including the Vena Tech--LGM vena cava filter, the Bird's Nest vena cava filter, and the Simon-Nitinol vena cava filter. The Vena Tech--LGM filter is a conical filter made from the PHYNOX alloy, with longitudinal stabilizing legs in addition to the intraluminal cone. The Bird's Nest filter is a "nest" of stainless steel wire which is wound into the vena cava, while the Simon Nitinol filter is a two-stage filter made from nickel-titanium alloy with a conical lower section and a petal-shaped upper section. All of these devices are permanent implants which cannot be removed from the body without a major surgical intervention.
Among numerous vena cava filters introduced in Europe but never brought to the United States was the optimal central trapping (OPCETRA) filter. The OPCETRA filter has two main parts: a main basket with ten, long stainless steel wire arms and a distal basket with five, short stainless steel wire arms. This design gives the filter an hourglass shape which provides a self-orienting structure for the filter within the lumen of a blood vessel. The OPCETRA filter was also a permanently implanted vena cava filter.
All of the above-identified vena cava filters are inserted into the body by passing the filter through a catheter to the site of deployment in the infrarenal inferior vena cava. After ejection from the catheter, these filters open or are manually deployed until the filter anchoring elements engage the vessel wall. These filters often have hooks or some other means by which the filter becomes fixed permanently to the vessel wall.
For an important subset of patients, in particular young trauma patients and patients undergoing total hip or knee replacement surgery, the risk of pulmonary embolism is short-term and limited to a definable period of time. Because of the long-term risks associated with implantation of a permanent vena cava filter, including venous stasis due to caval occlusion and its related complications, patients whose risk period is limited are not considered good candidates for permanent vena cava filters. The search for an appropriate temporary therapy for such patients lead to the development of temporary, tethered removable vena cava filters.
Tethered temporary vena cava filters are attached to a catheter and are implanted in the infrarenal vena cava with the tethering catheter extending out of the puncture site in the neck or groin, or buried subcutaneously within the soft tissues in the patient's neck. The tether remains coupled to the filter after deployment. The tether is then used to retrieve the filter. The potential for septic complications associated with the tethering catheter exiting the neck or groin require removal of such devices within fourteen days of placement. Risk periods for pulmonary embolism in such patients, however, can extend up to twenty-one weeks.
Temporary retrievable filters which are not attached to a tethering catheter have a construction similar to some versions of permanent filters. A hook or similar grasping structure is provided to allow a snare to engage the filter during the retrieval procedure. The filter in its entirety is then retrieved using a snare by drawing it into a catheter. However, to ensure the filter does not migrate within the vessel, barbs, anchors or similar structures must be used to engage the filter with the interior wall of the vessel for retaining it in place. These anchors make removal without injuring the vessel difficult. Moreover, after a relatively short period of time the portion of the filter legs in contact with the vessel wall are incorporated by endothelial tissue making retrieval difficult or impossible.
More recently, it has been proposed to provide a removable filter in two parts. An anchoring part of the filter engages the vessel walls, and become incorporated by endothelial tissue. A filter part is releasably coupled to the anchoring part. After the risk of embolism has passed, the filter part may be retrieved using a snare and catheter.
As of the time of the invention, there are no temporary or retrievable vena cava filters approved for use in the United States. The complications associated with permanent vena cava filters, tethered temporary filters and retrievable filters demonstrate that there remains a need for a filter device for use in treating the risk of embolism from the inferior venous system that overcomes the limitations of the present designs of permanent and temporary filter devices.